A method of the kind referred to above is disclosed in the article entitled "Measurement of gaseous oxygen using diode laser spectroscopy" in Applied Physics Letters, Volume 51, No. 18, Nov. 2, 1987, pages 1465 to 1467. The method refers to the gas spectroscopic measurement of oxygen concentrations on the basis of radiation absorption in the wavelength range of 760 to 770 nanometers. A laser diode with a monitor diode of type ML-4405 is used as a radiation source and has a wavelength range of 759 to 764 nanometers. The laser diode is supplied by a control current which is composed of a direct-current component and an alternating-current component having a frequency of 5 KHz. The operating point of the laser diode is adjusted with the direct-current component of the control current; whereas, the alternating-current component effects a periodic unbalance in the region of the absorption line. A sinusoidal-shaped curve trace is selected as the alternating current in order to drive the laser diode as harmonic free as possible. The laser diode and the monitor diode are mounted as a block on a temperature-controlled surface. The temperature of the surface brings the laser diode to one of the known absorption lines of oxygen.
The radiation emitted by the laser diode impinges on a detector unit after this radiation has passed through the gas to be investigated. The detector unit is connected to an evaluation circuit. The evaluation circuit includes essentially a lock-in amplifier having a signal input to which the measurement signal of the detector unit is applied and a reference input to which a signal voltage is applied having twice the frequency of the drive of the laser diode. Furthermore, the evaluation circuit includes a differential amplifier which likewise is connected to the measurement signal of the detector unit and receives a signal of the laser diode from the monitor diode which is proportional to the radiation power.
The output voltage of the differential amplifier corresponds to the absorption line for the measured oxygen concentration. The second derivative of the absorption line corresponds to the output signal of the lock-in amplifier. Since the absorption line is weakly pronounced especially for oxygen, this second derivative is utilized for the concentration measurement. The maximum amplitude of the output signal of the lock-in amplifier is approximately proportional to the partial pressure of the oxygen.
In the known method, it is disadvantageous that harmonic components from the current modulation of the laser diode occur because a non-linear relationship exists between the supplied radiation power and the control current of the laser diode. This non-linearity furthermore changes with the duration that the laser diode is used and effects an offset component in the output signal of the lock-in amplifier. In order to carry out a measurement, this offset, which is also dependent upon time, must be compensated by making an adjustment with a reference gas of known concentration. This is complex and affects the possibilities of utilizing a measuring system of this kind.
Published European patent application 0,183,761 discloses a photometric measuring arrangement wherein the radiation emanating from a measuring cell and from a reference measuring cell are received by separate detectors and the measurement signals of the detectors are supplied to an evaluation circuit. The detectors are additionally irradiated by a reference light source having modulated light whereby a second measuring signal is produced. The second measuring signal is, on the one hand, applied for making the detectors symmetrical in that the amplifier component of a detector is automatically tracked and, on the other hand, for adjusting the static radiation power of the reference light source. The radiation power of the reference light source is referred to a constant reference voltage source. The closed-loop control of the radiation power to a pregiven modulation profile is however not disclosed in this published European patent application.